1. Field of the Invention
The present invention relates to a method of reducing corrosion of a nuclear reactor structural material composed of a stainless steel or a nickel-base alloy, which is used in a nuclear power plant such as boiling water reactor (BWR).
2. Related Art
In a boiling water reactor (BWR) plant widely operated in the world, the cooling water contains a high concentration of oxidizing species or oxidizers such as oxygen and hydrogen peroxide which are generated by radiolysis of reactor water. Therefore, it is known that stress corrosion cracking (SCC) or intergranular stress corrosion cracking (IGSCC) occurs to a material such as a stainless steel or a nickel-base alloy used as a material constituting a nuclear reactor structure (which is called as a nuclear reactor structural material hereinlater) in the nuclear power plants. The generation of such SCC or IGSCC and the crack growth depend on the electrochemical corrosion potential (ECP). The electrochemical corrosion potential is decreased by reducing the concentration of oxygen and hydrogen peroxide, thereby suppressing the stress corrosion cracking and the crack growth.
In order to prevent such stress corrosion cracking (SCC) or intergranular stress corrosion cracking (IGSCC) (which may be merely referred to as SCC hereinlater), the following operations or techniques have been performed in nuclear power plants around the world.
That is, in a hydrogen water chemistry (HWC), a concentration of oxygen and hydrogen peroxide in a nuclear reactor water is reduced by injecting hydrogen in the feed water.
On the other hand, a noble metal such as Pt and Rh is deposited or adhered to the surface of a nuclear reactor structural material in advance, and the hydrogen is injected (see, for example, “Genshirosui kagaku hando bukku” (Handbook of nuclear reactor water chemistry) edited by Atomic Energy Society of Japan, Corona Publishing Co., Ltd., Dec. 27, 2000, p. 210, and Japanese Patent No. 2624906). In addition, for example, Japanese Unexamined Patent Application Publication No. HEI 07-270592 discloses an anticorrosion technique in which titanium oxide, which is known as a photocatalyst, is deposited to a material. Japanese Unexamined Patent Application Publication No. 2001-4789 discloses a technique combined a photocatalyst, a noble metal, and hydrogen injection.
In the known methods of reducing corrosion described above, the following inconveniences or problems have been provided.
For example, it is known that the reactor water in a nuclear reactor becomes a reducing condition by the hydrogen injection. The reactor water contains nitrogen compounds composed of radioactive nitrogen (N-16) generated by nuclear transformation of oxygen. These compounds including soluble substances such as a nitrate ion and a nitrite ion are changed into volatile ammonia under the reducing atmosphere in the reactor water. Unfortunately, the resultant ammonia flows into a main steam, thereby increasing the dose rate in the turbine system. Furthermore, since the injected hydrogen flows into an off-gas system, it is necessary to carry out a reaction to recombine the hydrogen with oxygen, and therefore, additional equipment is required.
On the other hand, the noble metal chemical addition is advantageous in that even a small amount of hydrogen injection can reduce the corrosion, compared with the hydrogen injection mentioned above. However, in order to allow a noble metal to adhere to a nuclear reactor structural material, a solution containing the noble metal must be injected in the nuclear reactor water. As a result, the noble metal also adheres to the surface of a fuel cladding tube composed of a zirconium alloy. This adhesion causes corrosion of the fuel material or increases the amount of hydrogen absorption.
In addition, at a portion to which the noble metal is deposited, when the hydrogen molar concentration is double or more of the oxygen molar concentration, the corrosion potential of the material drastically decreased. As a result, the material shows a very low potential, for example, −500 mV. Such a significant decrease in the corrosion potential impairs the stability of the oxide film formed on the surface of the material. As a result, radioactive metal oxides on the surface of the film or in the film are released in the reactor water.
Moreover, when the above noble metal chemical addition is performed in nuclear power plants, a large amount of noble metal adheres to a zirconium oxide film of the fuel. As a result, this adhesion increases the oxidation and hydrogenation of the fuel material. Furthermore, when the recombination of hydrogen with oxygen is performed on the surface of the noble metal, and the oxygen concentration in the reactor water is decreased, the dose rate in the turbine system is increased.
As described above, the noble metal chemical addition causes negative effects in the water quality conservation, the decrease in the flowing of radioactivity and the increase in burn-up of the fuel. In order to eliminate such negative effects, the development of a method of decreasing the injection amount of the noble metal or a method using an alternative substance of the noble metal is desired.
On the other hand, in the anticorrosion methods using a photocatalyst, the inconveniences or problems caused in the hydrogen injection or the noble metal chemical addition do not occur. However, as described in Japanese Unexamined Patent Application Publication No. HEI 07-270592, No. 2001-4789, and No. 2001-276628, in order to reduce the corrosion, SCC mitigation using a photocatalyst require light or radiation to excite the photocatalyst. Therefore, the application range of anticorrosion is limited to a structural material in the reactor such as a shroud, and unfortunately, a sufficient anticorrosion effect cannot be expected in other components such as primary loop recirculation system piping.